(1) Field of the Invention
The present invention relates to a method for the formation of buried gates of a semiconductor device, such as a static induction type thyristor or a gate turn-off thyristor, and in general to that of a buried gate type semiconductor device equipped with such buried gates.
(2) Description of the Prior Art
In a buried gate type semiconductor device, it is known that a faster switching speed is obtained at smaller gate resistance value. This is based on a fact that the electric signal transmission is effected through the buried gates. In this connection, it has been an important problem how to make the gate resistance value smaller.
The following two methods are the main ones among the known methods which have been used for the formation of buried gates in the buried gate type semiconductor devices.
(1) The gate is first formed by using diffusion method and thereafter the buried gates are formed by applying epitaxial growing on the surface thereof.
(2) Reentrant cut-in grooves are formed on top surface of a silicon substrate. Then the gates are formed by applying epitaxial growing in the grooves. The epitaxial growing is further applied above the gate surface and the buried gates are realized.
These known methods will be explained briefly for helping better understanding of the present invention by referring to the accompanying drawings.
FIGS. 1a to 1d are vertical cross-sectional views for briefly explaining the steps of formation of the buried gates by using the diffusion method. In view of convenience for the explanation, we may assume, as a practical embodiment that the substrate is N type and the gates are P type.
In FIG. 1a, reference numeral 1 designates the substrate, 2 is an oxide film and 3 is a window selectively provided in said oxide film.
In FIG. 1b, reference numeral 4 designate a gate formed by diffusion of P type impurity through the window 3.
FIG. 1c shows a step of removing the oxide film after the formation of the gate 4.
FIG. 1d shows a step of growing n-type silicon single crystal layer 5 on the surface having the gates 4 formed by epitaxial growing method. In this figure, CH illustrates a channel region forming a passage for the electric current.
When the gates 4 are formed by using the diffusion method as just has been described above, it is inevitably required to increase the surface concentration of the P type impurity at the time of diffusion of the gates. However, this may accompany the following disadvantages. For instance, in a known method, boron has widely been used as the impurity element for the diffusion of the P type gate. This is by a reason that boron has a masking effect for the oxide film and it produces a high surface concentration. But boron has relatively small atomic radius which is about 74% of that of silicon. Therefore, when such boron is diffused to obtain a high surface concentration of an order of 10.sup.19 -10.sup.20 (atoms/cc), crystal defects may be induced in lateral direction from the P type gate diffusion region, i.e. in the channel region CH. When n-type silicon single crystal layer having low impurity concentration of an order of 10.sup.14 -10.sup.15 (atoms/cc) is epitaxially grown on a surface including the abovementioned crystal defects, a phenomenon termed as auto-doping in epitaxial growth may occur. In this phenomenon, adjacent P-gates arranged in a very narrow interval decided by the device design may be short-circuited so that the corresponding channel region may be closed. As mentioned above, when the gates are formed by diffusion method, it is necessary to prevent occurrence of such closure of the channel region even by sacrificing the gate resistance. This has been a big drawback in the use of the diffusion method for the manufacture of the semiconductor devices having buried gates.
FIGS. 2a to 2d show steps of manufacturing semiconductor device using epitaxial growing method. These figures show corresponding steps with those shown in FIGS. 1a to 1d. In these FIGS. 2a to 2d, reference numeral 6 shows a cutting groove. Z illustrates an epitaxial growing layer. Namely, in FIG. 2a, 1' is the substrate and 2' is the oxide film. The cutting groove 6 is a groove formed in the substrate 1' and this groove can be formed easily by conventional manner through dry or wet etching using the oxide film 2' as the mask. FIG. 2b shows a condition in which the oxide film 2' of FIG. 2a is removed and then gates 4' are formed by applying P-type epitaxial growing on the surface having the cutting grooves 6 and thereafter the P-type epitaxial layer is further deposited thereon. By applying mirror polishing to the surface of the P-type epitaxial grown layer Z, a product shown in FIG. 2c can be obtained. Thereafter, by growing n-type silicon single crystal layer 5' on to the top surface having gates 4' formed, the buried gates are completed as shown in FIG. 2d.
In accordance with the abovementioned epitaxial growing method, the gate resistance can be made smaller since the distribution of the impurity concentration in the gates 4' is relatively uniform. For instance, if we assume that the impurity density of this gate 4' is the same order of the surface impurity density of the gate 4 manufactured in the steps shown in FIGS. 1a to 1d, the gate resistance obtained through the epitaxial growing method is smaller by about (1/5) to (1/10) compared with that obtained through the diffusion method. The reason for this is based on the fact that in the diffusion method, the impurity concentration distribution decreases exponentially from the surface towards the bottom. Accordingly, when observed from a view point to decrease the gate resistance, the epitaxial growing method is more beneficial. However, this epitaxial growing method has still drawbacks in that the reverse characteristics are not good, for instance, to result an increase of the leakage current at gate junction due to the fact that the crystallization is not complete at the boundary surface of the gate and the substrate since the epitaxial growing is applied in the deep cut groove as mentioned above. If the impurity concentration is made higher uniformly, channel closure may be caused at the time of the epitaxial growing. As the result, the epitaxial growing method is not satisfactory to manufacture the products in the commercial base.